Only approximately 50% of the radiation reaching the surface is photosynthetically active radiation (PAR). PAR is construed to comprise the wavelength region between 300 nm and 800 nm of the electromagnetic spectrum. Photosynthesis together with photoperiodism, phototropism and photomorphogenesis are the four representative processes related to interaction between radiation and plants. The following expression shows the simplified chemical equation of photosynthesis:6H2O+6CO2(+photon energy)→C6H12O6+6O2 
The typical absorption spectra of the most common photosynthetic and photo-morphogenetic photoreceptors, such as chlorophyll a, chlorophyll b and betacarotene, and the two interconvertable forms of phytochromes (Pfr and Pr) are presented in FIG. 1A.
The photomorphogenetic responses, contrary to photosynthesis, can be achieved with extremely low light quantities. The different types of photosynthetic and photo-morphogenetic photoreceptors can be grouped in at least three known photosystems: photosynthetic, phytochrome and cryptochrome or blue/UV-A (ultraviolet-A).
In the photosynthetic photosystem, the existing pigments are chlorophylls and carotenoids. Chlorophylls are located in the chloroplasts' thylakoids located in the leaf mesophyll cells of plants. The quantity or the energy of the radiation is the most significant aspect, since the activity of those pigments is closely related to the light harvest. The two most important absorption peaks of chlorophyll are located in the red and blue regions from 625 to 675 nm and from 425 to 475 nm, respectively. Additionally, there are also other localized peaks at near-UV (300-400 nm) and in the far-red region (700-800 nm). Carotenoids such as xanthophylls and carotenes are located in the chromoplast plastid organelles on plant cells and absorb mainly in the blue region.
The phytochrome photosystem includes the two interconvertable forms of phytochromes, Pr and Pfr, which have their sensitivity peaks in the red at 660 nm and in the far-red at 730 nm, respectively. Photomorphogenetic responses mediated by phytochromes are usually related to the sensing of the light quality through the red (R) to far-red (FR) ratio (R/FR). The importance of phytochromes can be evaluated by the different physiological responses where they are involved, such as leaf expansion, neighbour perception, shade avoidance, stem elongation, seed germination and flowering induction. Although shade-avoidance response is usually controlled by phytochromes through the sensing of R/FR ratio, the blue-light and PAR level is also involved in the related adaptive morphological responses.
Blue- and UV-A (ultraviolet A)-sensitive photoreceptors are found in the cryptochrome photosystem. Blue light absorbing pigments include both cryptochrome and phototropins. They are involved in several different tasks, such as monitoring the quality, quantity, direction and periodicity of the light. The different groups of blue- and UV-A-sensitive photoreceptors mediate important morphological responses such as endogenous rhythms, organ orientation, stem elongation and stomatal opening, germination, leaf expansion, root growth and phototropism. Phototropins regulate the pigment content and the positioning of photosynthetic organs and organelles in order to optimize the light harvest and photoinhibition. As with exposure to continuous far-red radiation, blue light also promotes flowering through the mediation of cryptochromes photoreceptors. Moreover, blue-light-sensitive photoreceptors (e.g. flavins and carotenoids) are also sensitive to the near-ultraviolet radiation, where a localized sensitivity peak can be found at around 370 nm.
Cryptochromes are not only common to all plant species. Cryptochromes mediate a variety of light responses, including the entrainment of the circadian rhythms in flowering plants such as the Arabidopsis. Although radiation of wavelengths below 300 nm can be highly harmful to the chemical bonds of molecules and to DNA structure, plants absorb radiation in this region also. The quality of radiation within the PAR region may be important to reduce the destructive effects of UV radiation. These photoreceptors are the most investigated and therefore their role in control of photosynthesis and growth is known reasonably well. However, there is evidence of the existence of other photoreceptors, the activity of which may have an important role in mediating important physiological responses in plants. Additionally, the interaction and the nature of interdependence between certain groups of receptors are not well understood.
Many plants can be grown in a different geographical location to their natural habitat by means of greenhouse cultivation utilising artificial light. It is known from WO 2010/053341 A1 by Zukauskas et al. that light emitting diodes (LEDs) can be used with phosphor conversion to satisfy some of the photomorphogenetic needs of plants. Phosphor conversion operates so that there is a light such as an LED that emits at a short wavelength adjacent to a phosphor component that absorbs and re-emits the radiation at a longer wavelength. This way the aggregate emission spectrum of the lighting device can be tuned, so that the photons provided to the plant allow the plant to grow in a certain way, e.g. to meet some morphological objectives such as stem height. This document is cited here as reference.
Light emitting diodes (LEDs) are increasing in popularity every day. A peculiar new structure used for LEDs is the quantum dot that is a semiconductor whose excitons are confined in all three spatial dimensions. Quantum dots have been suggested to be used to get rid of phosphor in WO 2009/048425 that discusses a multiple quantum well structure (MQW) comprising quantum dots. According to this publication, the MQW structure can be used to produce a phosphor free red and white nitride based LED. This document is also cited here as reference.
The prior art has considerable disadvantages. The prior art LEDs and phosphor arrangements do not allow sufficiently high resolution tuning of the emission spectra. The prior art MQW and quantum dot illumination devices are mainly focused on replacement of disadvantageous architectural features (such as phosphorus), which is of little help to a horticulturalist.
Quite clearly more sophisticated plant illumination technologies are needed that do more than merely replace unwanted technological features, i.e. enhance cultivation productivity also.